Knee probe having reduced thickness section for control of scrub motion

ABSTRACT

An improved knee probe for probing electrical devices and circuits is provided. The improved knee probe has a reduced thickness section to alter the mechanical behavior of the probe when contact is made. The reduced thickness section of the probe makes it easier to deflect the probe vertically when contact is made. This increased ease of vertical deflection tends to reduce the horizontal contact force component responsible for the scrub motion, thereby decreasing scrub length. Here “thickness” is the probe thickness in the deflection plane of the probe (i.e., the plane in which the probe knee lies). The reduced thickness probe section provides increased design flexibility for controlling scrub motion, especially in combination with other probe parameters affecting the scrub motion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. patent application Ser. No.11/450,977, filed on Jun. 9, 2006, entitled “Knee Probe having IncreasedScrub Motion”. Application Ser. No. 11/450,977 claims the benefit ofU.S. patent application Ser. No. 10/850,921, filed on May 21, 2004,entitled “Freely Deflecting Knee Probe with Controlled Scrub Motion”.

FIELD OF THE INVENTION

This invention relates to probes for testing electronic circuits ordevices.

BACKGROUND

Testing of electrical devices and circuits has been an importantcomponent of electronic manufacturing processes for some time. Suchtesting typically entails probing a circuit with a fixture includingmultiple flexible probes, each probe making electrical contact to acontact pad on the circuit chip. Various practical issues that havearisen in this context have been addressed in the prior art, especiallyin connection with providing reliable, low-resistance electricalcontact.

It is well known that electrical contact between the probe and thecontact pad can be hampered by the presence of non-conductive materialon the pad and/or the probe (e.g., a thin oxide film). Accordingly,considerable attention has been devoted to details of how the tip of theprobe moves relative to the contact pad in order to improve theresulting electrical connection. This relative motion is usuallyreferred to as a scrub motion. For example, U.S. Pat. No. 5,923,178considers a probe having a shape which provides a scrub motion which isprimarily a rocking motion without appreciable sliding. U.S. Pat. No.5,952,843 considers a probe having a canted tip portion to facilitatepenetration of the passivation layer. U.S. Pat. No. 6,529,021 considersa probe arrangement where the probe tip can be made to move in areciprocating manner to reduce contact resistance.

As circuit manufacturing technology continues to evolve to smallercritical dimensions, new practical issues relating to probing tend toarise which are not fully addressed by prior art approaches. Forexample, the decreasing size of contact pads as critical dimensions arereduced leads to increasingly demanding requirements on the ability toprecisely control the probe scrub motion. Excessive scrub motion cancause loss of electrical contact, if the probe moves off the contactpad.

Accordingly, it would be an advance in the art to provide greatercontrol of probe scrub motion.

SUMMARY

To better appreciate the present invention, it is helpful to considersome aspects of prior work by the present inventor. In particular, U.S.patent application Ser. No. 11/450,977 by the present inventor considersa knee probe where the knee curves away from the probe axis and thencurves back to connect to the probe tip, crossing the probe axis on theway to the tip. This configuration can be described as having a negativetip offset, in contrast to probes having no tip offset (i.e., the probetip is aligned with the probe axis), or probes having a positive tipoffset (i.e., the knee section does not cross the probe axis). Otherparameters being comparable, probes having negative tip offset tend toprovide longer scrub marks than probes having zero or positive tipoffset. In some cases, it is desirable to decrease the scrub lengthprovided by a probe having negative tip offset.

Such reduction in scrub length can be provided according to the presentinvention by modifying the probe shape. More specifically, the probeknee section includes a reduced thickness section to alter themechanical behavior of the probe when contact is made. Providing areduced thickness section of the probe makes it easier to deflect theprobe vertically when contact is made. This increased ease of verticaldeflection tends to reduce the horizontal contact force componentresponsible for the scrub motion, thereby decreasing scrub length. Here“thickness” is the probe thickness in the deflection plane of the probe(i.e., the plane in which the probe knee lies).

The reduced thickness section of the probe can be described in terms ofa probe thickness function h(z), where z is distance along the probe,having a local minimum. A probe having uniform thickness would have aconstant h(z), and a tapered probe would have a monotonically decreasingh(z). In either of these two conventional cases, h(z) would not have alocal minimum.

Although reduction of scrub length for negative tip offset probes is oneapplication of the invention, the invention is also applicable to probeshaving no tip offset and to positive tip offset probes. In general,embodiments of the invention can provide improved control of scrubmotion (e.g., by varying details of the reduced thickness section suchas location, amount of thickness reduction, etc.), especially incombination with other probe parameters affecting scrub motion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a probe according to a first embodiment of the invention.

FIGS. 2-3 show alternate embodiments of the invention having differenttip offsets.

FIGS. 4-5 show alternate embodiments of the invention having differentupper knee section thickness profiles.

FIG. 6 shows an embodiment of the invention having a tapered lower kneesection.

FIG. 7 a shows an embodiment of the invention in an initial contactconfiguration.

FIG. 7 b shows an embodiment of the invention in an operating contactconfiguration.

FIG. 8 shows a probe according to another embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 shows a probe 100 according to a first embodiment of theinvention. A shank 102 defines a probe axis 116. A curved knee section104 is connected to shank 102 and includes an upper knee section 106 anda lower knee section 108. A probe tip 112 is connected to an end of kneesection 104 opposite from the shank. Upper knee section 106 extendsoutward from shank 102 and reaches a knee point 110 of maximumseparation from probe axis 116, thereby defining a lateral knee offset118 from the probe axis. Lower knee section 108 extends from knee point110 toward probe axis 116 and to a tip location 114, thereby defining alateral tip offset 120 from the probe axis.

A probe plane includes and is thereby defined by probe axis 116 and kneepoint 110. In this example, the plane of FIG. 1 is the probe plane. Athickness of upper knee section 106 in the probe plane varies along thelength of the upper knee section according to a thickness function h(z),where z is position along the probe. The upper knee section includes areduced thickness section, as described above. More specifically, h(z)has a local minimum located between shank 102 and knee point 110, asshown in the example of FIG. 1.

Suitable materials for shank 102, knee section 104 and probe tip 112 arewell known in the art, and any such materials can be employed inpracticing the invention. Suitable tip materials are electricallyconductive and wear-resistant, and include Rh and Cr. Known probefabrication methods are applicable for fabricating embodiments of theinvention. These methods include, but are not limited to, standardmulti-layer metal deposition techniques such as plating, sputtering,photolithographic techniques and microelectromechanical systems (MEMS)techniques. No unusual fabrication difficulties due to the reducedthickness section arise in fabricating probes according to theinvention.

Preferably, h(z) varies smoothly (i.e., h(z) is preferably continuouswith a continuous first derivative) to avoid stress concentration atdiscontinuities and/or sharp corners of h(z). It is also preferred forthe minimum probe thickness (i.e., the local minimum of h(z)) to have avalue between about 0.5 h_(nom) and about 0.95 h_(nom), where h_(nom) isa nominal in-plane probe thickness. In some cases (e.g., as on FIG. 1),lower probe section 108 has a roughly constant thickness h₁, and in suchcases, h_(nom) can equal h₁. In other cases, the nominal probe thicknessh_(nom) can be taken to be the maximum value of h(z) (i.e., the maximumthickness of the upper knee section). In either of these two cases, thenominal in-plane probe thickness h_(nom) is typically between about 25μm and about 55 μm, although the invention can also be practiced outsideof this thickness range.

Preferably, h(z) varies smoothly along the entire length of upper kneesection 106, in order to minimize stress concentration for a givenminimum thickness. It is also preferred for the probe thicknessperpendicular to the probe plane to be somewhat higher than the nominalin-plane probe thickness, so that deformation of the probe is easiest inthe probe plane. More specifically, the out of plane thickness ispreferably between about 1.1 h_(nom) and about 1.5 h_(nom).

For a configuration with a small knee offset and large tip offset onecan expect a longer scrub length. For a configuration with large kneeoffset and small tip offset, a shorter scrub length is expected.Preferred probe design approaches depend on the friction between probeand contact pad. For large contact friction, probe designs that generatelarger horizontal reaction force typically produced with larger scrublength are preferred. For smoother, less frictional contact padsurfaces, probe designs producing a shorter scrub length are preferred.

Preferably, the reduced thickness section is in the upper knee sectionas shown and described above, although the invention can also bepracticed by having the reduced thickness section anywhere along thelength of knee section 104. Placing the reduced thickness section in theupper knee section tends to decrease scrub motion without appreciablydecreasing the contact force, while placing the reduced thicknesssection in the lower knee section (e.g., as shown on FIG. 8) tends todecrease both scrub motion and contact force. More specifically, anegative tip offset probe having an upper knee section reduced thicknesssection tends to rotate toward the knee during deflection, therebydecreasing scrub motion. A probe having a lower knee section reducedthickness section tends to have increased flexibility (which reducescontact force). This reduced horizontal scrubbing force decreases thescrub motion. Probes having multiple reduced thickness sections can alsobe employed (e.g., one being in the upper knee section and the otherbeing in the lower knee section) in practicing the invention.

As described above, the invention is applicable to probes having apositive tip offset, a negative tip offset, or no tip offset. Theexample of FIG. 1 shows a probe having a negative tip offset. Here probeaxis 116 is between knee point 110 and tip location 114. FIG. 2 shows anembodiment of the invention having no tip offset. Here tip location 114is substantially on probe axis 116. FIG. 3 shows an embodiment of theinvention having a positive tip offset. Here tip location 114 is betweenprobe axis 116 and knee point 110. For the probe of FIG. 3, the kneesection does not cross the probe axis.

Reduced thickness sections of probes according to the invention can beregarded as resulting from removing material from the left and/or rightsides of a smooth, constant-thickness probe profile. For example, FIG. 4shows an embodiment of the invention where the reduced thickness sectionis formed by variation of a right probe boundary f₂(z). FIG. 5 shows anembodiment of the invention where the reduced thickness section isformed by variation of a left probe boundary f₁(z). FIG. 1 shows anembodiment of the invention where the reduced thickness section isformed by variation of both a left probe boundary f₁(z) and a rightprobe boundary f₂(z).

In the preceding examples, lower knee section 108 has a roughly constantin-plane thickness. The detailed shape of lower knee section 108 is notcritical in practicing the invention, and any other lower knee sectionshape can also be employed in practicing the invention. For example,FIG. 6 shows an embodiment of the invention having a tapered lower kneesection 108. More specifically, the in-plane thickness of lower kneesection 108 decreases monotonically along the length of the lower kneesection from knee point 110 to the tip location 114.

Operation of the invention can be appreciated in connection with FIGS. 7a-b, which show an embodiment of the invention in an initial contactconfiguration and an operating contact configuration respectively. OnFIG. 7 a a probe according to the invention makes initial contact with acontact pad 704. It is convenient to describe the initial contact pointbetween the probe and contact pad 704 in terms of an initial contactoffset 708 defined with respect to probe axis 116. In operation, thearrangement of FIG. 7 a is vertically compressed (e.g., by movingcontact pad 704 up by a vertical deflection 706). Under thiscompression, the probe elastically deforms as schematically shown onFIG. 7 b. As a result of this deformation, the probe tip moves relativeto contact pad 704. Typically this relative motion includes atranslation (i.e., operating contact offset 708′ being different frominitial contact offset 708) and a rocking motion of the probe tiprelative to the contact pad surface. The rocking motion can beappreciated by noting the different orientations of a tip axis 702 onFIGS. 7 a and 7 b. The scrub mark length is the difference betweenoffset 708 and offset 708′

In one example, a reference probe (probe A) having a nominal in-planeprobe thickness of 52 μm and a negative tip offset (as shown on FIG. 1),provided a 25 μm scrub length on an A1 surface for 75 μm verticaldeflection. A probe according to the invention (probe B) had the sameshape as the reference probe, except that the upper knee section ofprobe B smoothly varied to provide a local minimum thickness of 33 μm inthe upper knee section. This local minimum was located about halfwaybetween the knee point and the shank. The thickness variation of theupper knee section was distributed over the entire length of the upperknee section. Probe B provided a 10 μm scrub length on the same Alsurface used for testing probe A. For both probes A and B, the contactforce was about the same (2 grams per 25 μm vertical deflection).

1. A probe for probing a device under test, the probe comprising: ashank defining a probe axis; a curved knee section connected to theshank and including an upper knee section and a lower knee section; aprobe tip connected to an end of the knee section opposite from theshank; wherein the upper knee section extends outward from the shank andreaches a knee point of maximum separation from the probe axis, therebydefining a lateral knee offset from the probe axis; wherein the lowerknee section extends from the knee point to a tip location, therebydefining a lateral tip offset from the probe axis; wherein a probe planeis defined by the probe axis and the knee point; wherein a thickness ofthe knee section in the probe plane varies along the length of the kneesection according to a thickness function h(z); wherein h(z) has a localminimum between the shank and the tip location.
 2. The probe of claim 1,wherein said probe axis is between said knee point and said tiplocation.
 3. The probe of claim 1, wherein said tip location is betweensaid knee point and said probe axis.
 4. The probe of claim 1, whereinsaid tip location is substantially on said probe axis.
 5. The probe ofclaim 1, wherein said thickness function h(z) is continuous.
 6. Theprobe of claim 1, wherein said thickness function h(z) is the separationbetween a left probe boundary f₁(z) and a right probe boundary f₂(z). 7.The probe of claim 6, wherein variation of said thickness function h(z)is substantially due to variation of said left probe boundary f₁(z) 8.The probe of claim 6, wherein variation of said thickness function h(z)is substantially due to variation of said right probe boundary f₂(z). 9.The probe of claim 6, wherein variation of said thickness function h(z)is due to both variation of said left probe boundary f₁(z) and tovariation of said right probe boundary f₂(z).
 10. The probe of claim 1,wherein said thickness function h(z) has a maximum value h_(max), andwherein said local minimum is between about 0.5 h_(max) and about 0.95h_(max).
 11. The probe of claim 1, wherein said thickness function h(z)has a maximum value h_(max), and wherein h_(max) is between about 25 μmand about 55 μm.
 12. The probe of claim 1, wherein a thickness of saidlower knee section in the probe plane decreases monotonically along thelength of the lower knee section from said knee point to said tiplocation.
 13. The probe of claim 1, wherein a thickness of said lowerknee section in the probe plane is substantially equal to a constant h₁along the length of the lower knee section.
 14. The probe of claim 13,wherein said local minimum is between about 0.5 h₁ and about 0.95 h₁.15. The probe of claim 13, wherein h₁ is between about 25 μm and about55 μm.
 16. The probe of claim 1, wherein said local minimum of h(z) isdisposed between said shank and said knee point.
 17. The probe of claim1, wherein said local minimum of h(z) is disposed between said kneepoint and said tip location.
 18. A method for probing a device undertest, the method comprising: a) providing a probe including: a shankdefining a probe axis; a curved knee section connected to the shank andincluding an upper knee section and a lower knee section; a probe tipconnected to an end of the knee section opposite from the shank; whereinthe upper knee section extends outward from the shank and reaches a kneepoint of maximum separation from the probe axis, thereby defining alateral knee offset from the probe axis; wherein the lower knee sectionextends from the knee point to a tip location, thereby defining alateral tip offset from the probe axis; wherein a probe plane is definedby the probe axis and the knee point; wherein a thickness of the upperknee section in the probe plane varies along the length of the upperknee section according to a thickness function h(z); wherein h(z) has alocal minimum between the shank and the knee point; b) making electricalcontact with the probe to a contact pad of the device under test.